In recent years, as the screen size of liquid crystal televisions, etc. has increased, more stringent demands have been placed on the display properties of optical films such as polarizing plates for use in image display devices such as liquid crystal displays, and more strict control of optical defects has been demanded. To meet these demands, there have been used methods of detecting optical defects with human eyes or automatic inspection apparatuses and removing them. When defects are detected, the information about their positions can be recorded, and the parts including defects can be eliminated so as not to be bonded to an optical display unit, so that an image display device as an end product can be prevented from being contaminated with optical film-derived defects. This can reduce the optical film loss and the rate at which reworking has to be performed to peel off the defective optical film from the optical display unit and therefore can contribute to an increase in yield and cost savings.
Proposed methods for recording positional information about detected defects include a method of printing a marking at a defect site generally with ink or the like (see for example Patent Document 1), a method of printing, on a film, bar code information or coded character information about the result of detection of defects (see for example Patent Document 2), and a method of attaching a recorded media such as an IC chip to an optical film product (see for example Patent Document 3).
When printing on an optical film is performed as described in Patent Document 1 or 2, the printed site has an increased thickness due to the thickness of the ink or the like used to form the marking, although the increase in thickness is very small. Therefore, the process of winding the optical film into a roll may cause a problem in which the thickness difference at the printed site, specifically a bump/dent deformation is transferred to a part in contact with the printed site, so that a new deformation such as a dent can occur to form a new defect in an originally-non-defective part. To prevent such transfer-induced defects, there is proposed a method of printing on a transverse end of an optical film outside the effective width of the optical film, specifically on a part not to be bonded to an optical display unit such as a liquid crystal cell (see for example Patent Document 4).
In addition to the demands for defect reduction, demands for reductions in the cost of image display devices are increasing every year. To meet the demands, there is proposed a method of manufacturing an image display device using a continuous manufacturing line that includes feeding means for feeding a long optical film from a roll of the long optical film, cutting means for cutting the optical film at specific intervals along the feed direction, and bonding means for bonding the cut piece of the optical film to an optical display unit (see Patent Document 5). Such a system makes it possible to directly cut the optical film from the material roll into the desired size and to directly bond the cut piece of the optical film to the optical display unit. Therefore, such a system makes it possible to directly package the material roll and to deliver it and therefore can contribute to a reduction in the number of man-hours or process materials, in contrast to a conventional method including stamping a sheet material of an optical film into pieces of the desired size, carefully packaging each piece of the optical film after the stamping, and delivering it to a panel processing manufacturer.
If the manufacturing method disclosed in Patent Document 5 is controlled in such a manner that the optical film is inspected for defects before it is cut by the cutting means and that defective parts of the optical film are not bonded to the optical display unit, the yield of the end product, an image display device, can be increased.
Patent Document 5 proposes that in the defect inspection, the optical film should be inspected for defects after a release film (separator) is temporarily peeled off from the optical film, and then a separator should be attached to the optical film again. Alternatively, detect inspection may be performed in advance, and the resulting defect information may be recorded on the material roll as described in Patent Documents 1 to 4, so that the optical film from the material roll can be directly cut into the desired size without peeling off the separator and that the cut piece of the optical film can be directly bonded to the optical display unit.